In digital printing, data relating to an image, or the images of a multi-page document, originates in a host computer and is sent to a digital printer. The digital printer typically includes what can be called a “software” component, and a “hardware” component including a marking engine, which responds to signals to place marks on a sheet consistent with the image data. The software component typically receives image data form the host computer in a highly specialized format, such as JPEG or in a page description language such as a PDL. Typically, color information is retained by the host computer in a device-independent format such as L*a*b* color space, and then converted to a machine-operable format such as CYMK through one or more look-up tables associated with the printer. The software component within the printer interprets the received image data and thereby converts the data into a format more directly operative of the hardware, such as in a relatively simple compression format. The data that operates the hardware influences the hardware such as to cause a laser or an ink-jet printhead to operate at a particular time as a sheet is being fed through the hardware.
In addition, the marking engine, whether xerographic, ink-jet, or of some other type, has associated therewith any number of control systems to maintain a desired print quality, as is generally known in the art. Certain control systems influence specific physical conditions of the engine, such as, in the xerographic case, energy levels associated with charging, exposure, and/or development. In a basic case, the control systems operate to cause the printer hardware to approach an ideal print quality output that relates to the input image data sent from the host computer.
An emerging trend in digital printing is parallel printing, which can be defined as operating a plurality of printers to output a set of printed sheets which can subsequently be combined, such as by binding or otherwise; or for which it is generally desired that there be consistency in print appearance. In such a case, and especially with color printing, it becomes desirable to have all of the printers output prints which appear to be consistent with each other, such as in color palette, line width, and other attributes. Whereas with a single printer, the control system is designed to approach an ideal of the original data, when multiple printers are involved, the approach to the original image data is to some extent overruled by the need to make the outputs of several printers consistent: the need for consistency may require that one or all printers to some extent output “sub-optimal” print quality.
U.S. Pat. No. 6,157,735 is largely directed to the problem of maintaining a consistent color output among multiple printers. In the described system, “color calibration data” is transmitted variously around a network of printers, spectrophotometers, and computers. However, this color calibration data is purely on the software level, largely concentrating on the conversion of color data to device-dependent space.
U.S. Pat. No. 6,091,518 shows another system in which “color correction data” is passed over a network.